Some actors have been around for so long, and have done so much solid work in so many genres, that’s it’s easy to take for granted just how terrific they really are. Donald Sutherland, who turns 79 on July 17, is one such actor. For almost 50 years, he’s been bringing his unique, engaging intelligence (emotional, and the other kind) to bear on roles so phenomenally diverse that his filmography often reads like a compendium of several actors’ careers.

There were the early films, before he became one of Hollywood’s unlikeliest leading men: The Dirty Dozen, for example, and the costume comedy, Start the Revolution Without Me — two movies that, in every possible way, could not be more different.

Then came the Seventies, and the movies and roles that made the Canadian native a bona fide, if un-pigeonhole-able, star: Hawkeye Pierce in Robert Altman’s M*A*S*H. The title character in Klute (playing opposite Jane Fonda, who won her first Oscar for her searing portrait of a call girl—and with whom Sutherland had a brief, intense off-screen romance). Nicolas Roeg’s freaky cult favorite, Don’t Look Now, with Julie Christie at her most preposterously beautiful. His astonishing turn as Homer Simpson (not that Homer Simpson) in John Schlesinger’s uneven, riveting 1975 adaptation of Nathanael West’s classic short novel, Day of the Locust, about the violence and howling despair always lurking just beneath Hollywood’s glittering surface.

And, of course, the 1978 remake that surpassed the classic sci-fi original, Invasion of the Body Snatchers.

In the ’80s, the ’90s and the 2000s, Sutherland kept working, starring in movies of wildly varying quality, but always leaving a mark—sometimes with an entire performance; sometimes with a gesture; sometimes with a wholly unexpected (but perfect) delivery of a line and an arching of those famous eyebrows. Ordinary People, Eye of the Needle, Six Degrees of Separation, Pride and Prejudice, right up to The Hunger Games, with notable TV work (Dirty Sexy Money, Oldest Living Confederate Widow Tells All and many more) thrown in for good measure.

One gets the sense that not only can Donald Sutherland play absolutely any type of role offered to him, in any genre, but that at some point in the past five decades, he probably already has.

Here, on his 79th birthday and in honor of his long, stellar, still-vibrant career, LIFE.com offers a gallery of rare pictures (none of them ran in LIFE magazine) of Sutherland and his family, including son Kiefer, by photographer Co Rentmeester in 1970. He and his wife at the time, the Canadian activist and actress Shirley Douglas, divorced not long after these photos were made. But in their depiction of an already accomplished actor on the cusp of a transformative period in both his career and his personal life, Rentmeester’s photographs capture one of the movies’ singular, most versatile talents, quite literally letting his hair down.
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